地图模态层的实现

var boundCoord = [[[-180,-90], [180,-90], [180,90], [-180,90], [-180,-90]]];
var zoneCoord = [[[],[]]];
var boundGeo = turf.polygon(boundCoord),
    zoneGeo = turf.polygon(zoneCoord);
var modalJson = turf.difference(boundGeo, zoneGeo);

var features = (new ol.format.GeoJSON()).readFeatures(modalJson);
var vector = new ol.layer.Vector({
    renderMode: "image",//image, vector
    source: new ol.source.Vector({
        features: features
    }),
    style: new ol.style.Style({
         fill: new ol.style.Fill({
         color: 'rgba(0, 0, 0, 0.5)'
        })
     }),
    zIndex: 999
 });
 map.addLayer(vector);